1. Field of the Invention
The present invention relates to a semiconductor process, and more particularly, to a method of forming a contact hole on a semiconductor wafer.
2. Description of the Prior Art
In a multilevel metallization process, metallic conductive layers are used to connect each metal oxide semiconductor (MOS) transistor on the semiconductor wafer to form a stacked circuit structure. The multilevel metallization process improves the integration of the semiconductor wafer so it is widely applied in very large scale integration (VLSI) and Ultralarge-scale integration (ULSI) processing. The inter-layer dielectric (ILD) layer between the MOS transistor and the metallic conductive layer protects and insulates the electric units on the semiconductor wafer. The contact plug in the ILD layer functions as an electric conductive wire for the MOS transistor and the metallic conductive layer. In the formation of the contact plug, etching is performed to form the contact hole. Then the contact hole is filled with metal material to form the contact plug.
In semiconductor processing beyond 0.25 .mu.m, a borderless contact hole is used to increase the available surface area on the semiconductor wafer. The prior art contact etch process has a silicon nitride layer positioned between the ILD layer and the MOS transistor. The ILD layer is made of silicon oxide. The silicon nitride layer functions to prevent further etching. When producing the borderless contact hole on the ILD layer, the etching process stops at the silicon nitride layer. In this way, further etching of silicon oxide portions of the MOS transistor is prevented.
Please refer to FIG. l. FIG. 1 is a schematic drawing of the structure of the prior art borderless contact holes 24, 26 on the semiconductor wafer 10. The semiconductor wafer 10 comprises a substrate 12, a MOS transistor 14, a Ti-silicide layer 19, a shallow trench 16 positioned on the substrate 12, a silicon nitride layer 18 positioned on the MOS transistor 14, a silicon oxide layer 20 deposited on the silicon nitride layer 18, and a photo-resist layer 22 deposited on the silicon oxide layer 20 to define the position and pattern of the borderless contact holes 24, 26.
In the formation of the borderless contact holes 24,26, the silicon nitride layer 18 and the silicon oxide layer 20 are first deposited onto the semiconductor wafer 10. The photo-resist layer 22 is then formed in the predetermined region of the silicon oxide layer 20. This is accomplished by using the mask for pattern transfer thereby defining the position of the borderless contact holes 24,26. Next, the contact holes 24, 26 are grossly formed by performing an anisotropic dry etching on those portions of the silicon oxide layer 20 which are not covered by the photo-resist layer 22. The reactive gases used in dry etching are formed from a mixture of fluorocarbon, such as C.sub.4 F.sub.8 and Carbon monoxide (CO) with argon (Ar). CO is introduced to balance the ratio of the carbon atoms to fluorine atoms.
Then, the silicon oxide layer 20 is removed down to the silicon nitride layer 18. The etching process of the silicon nitride layer 18 and the photo-resist strip process are performed to complete the formation of the contact holes 24,26. In the dry etching process of the silicon oxide layer 20, polymers may form on the side walls of the contact holes 24,26 and the surface of silicon nitride layer 18. These polymers interfere with etching of the silicon nitride layer 18 and may also adversely affect the after etching inspection critical dimension (AEICD) of the borderless contact hole 24,26. Also, during the dry etching process of the silicon oxide layer 18, polymers remaining on the surface of the silicon nitride layer 18 may cause defects on the gate or the drain of the MOS transistor 14 under the contact hole 26 to occur.
In the dry etching process of the silicon nitride layer 18, some polymers form on the side walls and the base of the contact holes 24,26. Therefore, the contact area of the gate and drain of the MOS transistor 14 at the base of the contact holes 24,26 is reduced. When the contact hole is filled with metal material to form a contact plug, the reduced contact area increases the resistance (over 5 .OMEGA.) of the contact plug.